A conventional downhole rod pump incorporates a pair of ball and seat-type check valves, commonly referred to as a standing valve and a travelling valve. A conventional design for this long known valve is shown in FIG. 1.
The conventional valve comprises a bottom nose sub, a barrel and a top sub. These tubular components are screwed together end to end to form the outer housing of the valve. The innards of the valve comprise an annular seat member, a cylindrical cage, and a ball or plunger. The seat member is inserted into the bore of the barrel from the bottom, to abut an inwardly projecting ring shoulder formed by the barrel wall. The seat member is locked in place and supported from below by the nose sub. The seat member provides an annular valve seat against which the ball seals to close off the flow passage extending through the valve. The nose sub, seat member and barrel combine to form an outer bottom seal below the ball and seat seal. The cage is inserted from the top end of the barrel bore, seats on the barrel ring shoulder and is locked in place by an annular seal ring and the top sub. The sidewall of the cage typically forms four windows separated by the vertically extending non-removed portions which are referred to as guide ribs. At its upper end the cage has a transverse stop bar. The ball fits loosely in the cage, so that it can easily leave the seat and move up or down within the cage. However, its fit relative to the cage is close enough so that the ribs effectively guide it and prevent it from rattling around excessively in the cage. The stop bar limits the upward movement of the ball.
Now, one aspect of the present invention has to do with increasing the flow capacity of the valve, keeping in mind that the inner diameter of the barrel bore limits the radial dimensions of the valve innards. Stated otherwise, one is precluded from enlarging the outer diameter of the valve innards when one attempts to increase the flow capacity of the valve.
At this point, it is appropriate to identify the constraining flow areas within the valve. FIG. 7 schematically shows them. The flow areas are cross-hatched. More particularly, the valve flow passage comprises, from the bottom up:
the bore of the seat member; PA0 the space left unoccupied when the ball is positioned off the seat within the cage, said space being mainly provided by the windows; and PA0 the space left between the stop bar and the upper ring of the cage. PA0 A plunger comprising a valve member having an arcuate sealing surface and being relatively small in diameter, so that it is inwardly spaced from the cage ribs, thereby increasing the available flow area around the valve member when it is displaced off the seat and up into the cage chamber; PA0 Equipping the now smaller valve member with integral, axially extending, centrally positioned top and bottom stems that extend through aligned apertures in transverse webs carried by each of the seat member and cage. (The stems and valve member together make up the plunger.) The stems and webs cooperate to provide means for loosely guiding and centralizing the valve member in the course of its movement, so as to maintain a consistent path of travel and to ensure that the valve member consistently seats centrally and avoids rattling against the cage ribs (which leads to nicking of the valve member's seal surface); PA0 Since the stems reciprocate in the web apertures, it is desirable to minimize sticking and ensure that solids do not pack solidly between the parts, so that ease of movement is lost. Therefore each stem has a loose fit in its corresponding web and the apertures in the webs are circular and the stem is straight-sided, to create clearance for fluid flow between the two parts, so that sand particles and the like may be flushed out. By having an arcuate sealing surface and a loosely working guide means, the valve member is adapted to "work" its way down into the seat to effect a circumferential, line contact seal; PA0 The valve member of the plunger is semi-spherical in form and has a spacing neck at its upper end, to permit the upper guide web to be positioned part way down the cage (so that the top flow area is enlarged), while still enabling the valve member to move far enough off the seat to provide the desired clearance, and to leave some space between the valve member and the guide web when the valve is fully open; PA0 The cage is formed with only 2 or 3 ribs, to thereby widen the windows; and PA0 The valve member seal surface and seat are formed with complementary radius-on-radius or radius-on-bevel configurations, to ensure that a liquidtight, essentially linear circumferential contact seal is maintained, even though the stems and webs wear and the tolerance between them increases. PA0 the flow area through the valve has been increased; PA0 by guiding the valve member, it remains generally centered relative to the seat; PA0 by guiding the valve member, it cannot rattle in the cage and be nicked; and PA0 by using loose-fitting guide means and line contact sealing surfaces, the effective life of the seal has been maintained.
The question arises: Why would one want to increase the flow capacity of the valve and thus that of the pump itself? The present invention arose in connection with the pumping of heavy oil (although the valve is not limited to that application). A significant problem in pumping viscous heavy oil is that the rod string can only slowly drop down through the column of oil present within the tubing string in which the rod string reciprocates. So one is limited to a certain number of strokes per minute. If the well is capable of producing more fluid than the pump can move, given the limitation on stroking rate, then production is being hampered. In the alternative, if stroking rate can be increased, operating costs will increase as the system is driven harder.
So it has long been appreciated that it would be desirable to improve the flow capacity of the valves used in a given size of downhole pump. However it has also been a condition that if this is to be done, the change in design cannot be accompanied by a significant loss in the useful operating life of the valve.
Turning now to the patent literature known to applicant, the following references are of interest:
______________________________________ U.S. 1,353,409 - McEvoy U.S. 170,974 - Westwater U.S. 1,860,004 - Yardley U.S. 1,960,970 - Fina U.S. 2,233,649 - Stahl et al U.S. 2,943,639 - Smith U.S. 4,369,808 - Hagman ______________________________________